Pressure sensors are used in automotive vehicles for a variety of purposes, such as for sensing the vacuum pressure at the intake manifold of a vehicle's engine, as well as for other applications. Generally, such pressure sensors contain a pressure sensitive element. A common type of pressure sensitive element includes a piezoresistive silicon element which is integrated with appropriate adjusting circuitry in a monolithic silicon integrated circuit. The piezoresistive silicon element typically contains a diaphragm and piezoresistive strain sensors disposed on the diaphragm, which are connected to the adjusting circuitry for measuring the deflection of the diaphragm due to pressure changes.
Often, the pressure sensor requires calibration to compensate for the variations that tend to occur during manufacturing between individual pressure sensing elements and signal conditioning circuitry during manufacturing. To provide for this compensation, it is advantageous to include either separately or as part of the integrated circuit, a conditioning network that permits customized adjustment of the output parameters for the individual pressure sensor. This conditioning network typically is a network of resistors, of which some may be selectively modified or removed from the integrated circuit by laser trimming the area of the resistors or alternatively by opening fusible links in the network. However, in order to calibrate these pressure sensors, the critical pressure sensing element must first be enclosed within a pressurizable chamber.
In the past, many different types of pressure sensor assemblies have been disclosed. Often, the pressure sensing element has been disposed within a separate container for pressurization by means of a port, such as disclosed in U.S. Pat. No. 4,295,117 to Lake et al. The separate container allows the pressure sensing element to be appropriately pressurized while the signal conditioning circuitry remains exposed, thereby facilitating the laser trimming of the exposed circuitry during calibration. An illustrative example of this type of prior art pressure sensor assembly is cross-sectionally illustrated in FIG. 1. (The size of the components shown in FIGS. 1 through 3 are exaggerated for purposes of description.) The container 10 housing the pressure sensing integrated circuit is located adjacent to, but separate from, the substrate lo having the associated signal conditioning circuitry. The container 10 is required so that the pressure sensing element can be pressurized. Both the container 10 and substrate 16 are glued to a backplate 14, therefore leaving only one side of the substrate 16 available for population by the signal conditioning circuitry. This is an inefficient use of substrate space, and therefore unduly increasing the size of the assembly. During calibration of this type of sensor, pressure is applied to the pressure sensing element within the container 10 through a port 12. A laser is then able to access, from above, and trim the resistor 20 within the signal conditioning circuitry, before a cover 18 is attached.
This prior art approach of FIG. 1 has been satisfactory, although there are shortcomings. The separate container 10 housing the sensing element undesirably increases the number of components within the assembly. The use of a separate container 10 also requires that additional interface joints be formed within the assembly which increases the number of processing steps, thereby possibly decreasing product reliability. Also, as stated previously, by utilizing only a single side of the substrate 16 for the signal conditioning circuitry, the available area of the substrate is not efficiently maximized.
An alternative prior art approach is cross-sectionally shown in FIG. 2. In this configuration, the critical pressure sensing element 10a is attached to a substrate 16 having the associated signal conditioning circuitry also disposed thereon. The substrate 16 is attached to a backplate 14. The pressure sensing element 10a is not contained within a separate container. Rather, pressurization of only the backside of the pressure sensing element 10a is achieved by means of the port 12. This embodiment thus mandates that backside sensing of pressure changes are needed for calibration of a resistor 20 within the signal processing circuitry. After laser trimming, a cover 18 is appropriately attached to seal and protect the components.
This second approach is problematic in that, again, only a single side of the substrate 16 can be populated with signal conditioning circuitry, since the second side is necessarily sealed by the backplate 14 for pressurization of the pressure sensing element 10a through the port 12 during calibration. Therefore, the second side cannot be exposed to the laser trimming operations, leaving only the topside available for deposition of the signal conditioning circuitry. In addition, backside sensing technology is required.
Still another illustrative prior art approach is cross-sectionally shown in FIG. 3. Here, the substrate 16 is suspended within a housing 18, therefore permitting both sides of the substrate 16 to be populated signal conditioning circuitry. However, in order to calibrate the resistors 20a and 20b of the signal conditioning circuitry disposed on both sides of the substrate 16 by laser trimming, the resistors 20a and 20b must be exposed. This, therefore, requires that the pressure sensing integrated circuit be enclosed within its own container 10 and pressurized by means of the port 12, since the sealing cover 18 is not attached until after the laser trimming operations during calibration. Although this approach allows both sides of the substrate 16 to be populated by signal conditioning circuitry, the design is still less than ideal since the pressure sensing element requires a separate, pressurizable container 10. In addition, a design such as this would most probably include electrically conductive through-holes for electrical interconnection of the two sides of signal conditioning circuitry. For calibration, these through-holes would also require sealing unless the pressure sensing integrated circuit was sealed in its own container, as shown.
Lastly, U.S. Pat. Nos. 4,756,193 and 4,859,227 to Luettgen and Luettgen et al., respectively, have offered an alternative approach to housing the pressure sensing element within a separate container. Both Luettgen patents leave the pressure sensing chip in an open chamber and then enclose it by means of a flange on the pressure application nozzle. Therefore, when the chamber requires pressurization, such as for the calibration operation, the pressure sensing element is sealed by the flange of the pressure application nozzle. This has been a satisfactory approach; however, this design utilizes only a single side of the device for disposition of the signal conditioning circuitry.
Therefore, it would be most advantageous to provide a pressure sensor assembly which maximizes the efficiency of the design by employing a double sided substrate for population by the signal conditioning circuitry, and by eliminating the previous requirement for separately containing the pressure sensitive element. In addition, such a pressure sensor assembly should facilitate the calibration of the signal conditioning circuitry of the pressure sensing element by laser trimming operations. Lastly, it would be desirable if such a pressure sensor was amenable for manufacturing by automotive production techniques for use in automotive applications.